The zinc and zinc alloy coatings are highly susceptible to corrosion and therefore chromate passivation is usually applied directly after galvanizing in line to enhance the corrosion protection properties of zinc and zinc alloy. This method is widely applied as an effective and economical method of corrosion prevention. However, chromate conversion coating contains hexavalent chromium which is carcinogenic and they are environmentally and toxicologically hazardous. Therefore, alternative coating development for protecting zinc and zinc alloys is a growing need of industries.
In recent trend, thin organic protective coating formulation by sol-gel process has found a most suitable alternative to chromate passivation. In sol-gel method, small molecules can be converted into polymeric or ceramic materials depending upon the monomers selection. Silance sol-gel chemistry consists primarily of hydrolysis and condensation reactions of alkoxysilane precursors that form macro- or nanoporous gel as the reaction proceed. The hydrolysis reaction are typically catalyzed by base or acid conditions, producing partly or completely hydrolyzed silanol and then condensation reaction occur to form siloxane bridge resulting in a polymerization to form molecules of a glass-like oxide or ceramic network.
U.S. Pat. No. 6,162,498, has described a sol-gel coating formulation consisted of precursors methyltriethoxy silane, tetraethoxy silane, silica sol and then it was catalysed by orthophosphoric acid addition. This coating gives colourless coating formulation on cleaned substrate. However, this sol-gel formulation required filtration prior to application on substrate. Furthermore, after coating formulation it started to become viscous and then became gel very fast. Similarly the. U.S. Pat. No. 6,579,472 has described the uses of triethylphosphate as a preferable corrosion inhibitor but it was also not found suitable for zinc and zinc alloy steel substate.
The present invention is aimed to develop a sol-gel coating formulations on removing the above difficulties of prior art.
The present invention describes a sol-gel coating formulations which after application on zinc and zinc alloy coated substrate enhances the corrosion resistance properties of the substrate. The coating performance of the sol-gel formulation is further enhanced by addition of inorganic compound, particularly sodium metavanadate. The sol-gel coatings enhance the corrosion resistance as well as give a hydrophobic surface characteristic on metal surface.